Conventional tandem axle assemblies can include forward and rear axle assemblies and a drive shaft assembly connecting the two axle assemblies. The forward and rear axle assemblies each include a pair of axle half shafts extending therefrom on which one or more wheels of a vehicle are mounted. The forward and rear axle assemblies further include a differential gear set that allows the vehicle wheels on each axle assembly to rotate at different speeds. The differential gear set includes a pinion gear in mesh with a ring gear that drives a plurality of bevel gears to cause rotation of the axle half shafts. The pinion gears of the forward and rear axle assemblies are driven by an inter-axle differential housed within the forward axle assembly. The rear axle assembly is driven by the inter-axle differential through the drive shaft.
Vehicles with tandem axles are offered in multiple gear ratios depending on the requirements of the vehicle. To facilitate different gear ratios, the tooth combinations of the ring and pinon gears, amongst other gears, in both the forward and rear axle assemblies must be changed. The slower the desired axle ratio (i.e. the higher numerically), the faster the pinion gear must rotate for a given speed. Power consumption is a multiplication of torque and rotational speed, the power consumption increases as the desired axle ratio increases.
By adjusting the rotational speed of the pinion gears, the rotational speeds of other components, including bearings supporting the gears, must be adjusted. The bearings that support the pinion gears create an inordinate amount of drag as they rotate through lubricant. The parasitic power losses of the bearings is a function of speed due to the amount of parasitic fluid drag resulting from rotating through the lubricant. Power consumption is a function of the multiplication of torque and rotational speed. Thus, the pinion bearings consume more power the slower the axle gear ratio because the bearings rotate at a faster speed.
Therefore, it would be advantageous to develop a tandem drive axle system for a vehicle that reduces the power consumption of the axles including the bearings to improve the overall efficiency of the system and allows for adjusting the desired axle ratio by changing one set of gears.